So, from what I've gathered through the few online posts about this, there's no definitive answer to this problem. So I'll ask here, to be 100% sure. There is no solution to this big hole appearing beneath the player when fast flying? FAQ does mention holes, but I think I know what holes it's referring to (cause I may have this problem too) and the description is not this.
Here, I'm using Creative Fly, but sometimes it does that even while flying with the Elytra. This is single player, 1.21.8, 21 vanilla chunks, 8GB of memory allocated for MC and shaders are the latest version of MakeUp UltraFast (the situation does not change with or without shaders). Also, sometimes this lasts about 10-20 seconds before rendering it all, other times it seems to last forever.
These are all my other mods:
* Ambient Sounds
* AppleSkin
* Better Advancements
* BetterF3
* Better Third Person
* Concurrent Chunk Management Engine (C2ME)
* Cherished Worlds
* Cool Rain
* Creative Fly
* Do a Barrel Roll
* Easy Anvils
* Easy Magic
* Entity Culling
* Falling Leaves
* FallingTree
* FerriteCore
* Horseman
* Iris Shaders
* InvMove
* REI
* Lithium
* Mod Menu
* NoisiumForked
* Not Enough Animations
* Ore Discovery Broadcast
* Presence Footsteps
* Reese's Sodium Options
* Shulker Box Tooltip
* Sit
* Sodium
* Sodium Extra
* Sound Physics Remastered
* Tiny Item Animations
* Vanilla Refresh